Since the discovery of high-temperature superconducting (HTS) materials (superconducting above the liquid nitrogen temperature of 77 K) there have been efforts to develop various engineering applications using such HTS materials. In thin film superconductor devices and wires, the most progress has been made with fabrication of devices utilizing an oxide superconductor including yttrium, barium, copper and oxygen in the well-known basic composition of YBa2Cu3O7-x (hereinafter referred to as Y123). Progress has also been made with compositions containing rare earth elements (“RE”) partially substituted for Y. Biaxially textured superconducting metal oxides, such as Y123, have achieved high critical current densities in a coated conductor architecture. These wires, often referred to as second generation HTS wires, are the preferred material for many applications, including cables, motors, generators, synchronous condensers, transformers, current limiters, and magnet systems for military, high energy physics, materials processing, transportation and medical uses.
The current carrying capability of the HTS material is strongly related to its crystalline alignment or texture. The oxide superconductor grains typically are aligned with their c axis perpendicular to the plane of the wire surface and the ab plane parallel to the wire surface. Grain boundaries formed by the misalignment of neighboring crystalline HTS grains are known to form an obstacle to superconducting current flow, but this obstacle decreases with the increasing degree of alignment or texture. Therefore to make the material into a commercially viable product, e.g. an HTS wire, the HTS material must maintain a high degree of crystalline alignment or texture over relatively long distances. Otherwise, the superconducting current carrying capacity (critical current density) will be limited.
HTS materials can be fabricated with a high degree of crystallographic alignment or texture over large areas by growing a thin layer of the material epitaxially on top of a flexible tape-shaped substrate, fabricated so that it has a high degree of crystallographic texture at its surface. When the crystalline HTS material is grown epitaxially on this surface, the crystal alignment of the HTS material grows to match the texture of the substrate. In other words, the substrate texture provides a template for the epitaxial growth of the crystalline HTS material. Further, the substrate provides structural integrity to the HTS layer.
A substrate can be textured to provide a template that yields an epitaxial HTS layer. Materials such as nickel, copper, silver, iron, silver alloys, nickel alloys, iron alloys, stainless steel alloys, and copper alloys can be used, among others. The substrate can be textured using a deformation process, such as one involving rolling and recrystallization annealing the substrate. An example of such a process is the rolling-assisted biaxially textured substrate (RABiTS) process. In this case large quantities of metal can be processed economically by deformation processing and annealing and can achieve a high degree of texture.
One or more buffer layers can be deposited or grown on the substrate surface with suitable crystallographic template on which to grow the HTS material. Buffer layers also can provide the additional benefit of preventing diffusion of atoms from the substrate material into the crystalline lattice of the HTS material or of oxygen into the substrate material. This diffusion, or “poisoning,” can disrupt the crystalline alignment and thereby degrade the electrical properties of the HTS material. Buffer layers also can provide enhanced adhesion between the substrate and the HTS layer. Moreover, the buffer layer(s) can have a coefficient of thermal expansion that is well matched to that of the superconductor material. For implementation of the technology in commercial applications, where the wire may be subjected to stress, this feature is desirable because it can help prevent delamination of the HTS layer from the substrate.
Alternatively, a non-textured substrate such as Hastelloy can be used, and textured buffer layers deposited by means such as the ion-beam-assisted deposition (IBAD) or inclined substrate deposition (ISD). Additional buffer layers may be optionally deposited epitaxially on the IBAD or ISD layer to provide the final template for epitaxial deposition of an HTS layer.
By using a suitable combination of a substrate and one or more buffer layers as a template, an HTS layer can be grown epitaxially with excellent crystal alignment or texture, also having good adhesion to the template surface, and with a sufficient barrier to poisoning by atoms from the substrate. The HTS layer can be deposited by any of a variety of methods, including the metal-organic deposition (MOD) process, metal-organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), thermal or e-beam evaporation, or other appropriate methods. Lastly, a cap layer can be added to the multilayer assembly, which helps prevent contamination of and damage to the HTS layer from above. The cap layer can be, e.g., silver, and can be, e.g., sputtered onto the HTS layer.
HTS wire development continues to seek improvements in critical current density, in particular, critical current density in high magnetic fields and temperatures (Jc(H,T)). This improvement can come by improving the “pinning” of the superconducting vortices, which is the underlying mechanism for high critical current density Jc in HTS materials. To achieve pinning in superconductors, local potential energy differences should be matched in size as closely as possible to the size of the normal core of the superconducting flux line or vortex. The cross-sectional core has a size on the order of the coherence length, which is several nanometers in high temperature superconducting cuprates and grows with temperature. Thus, nanometer-sized defects are introduced into the oxide superconductor grains to pin flux lines and improve current carrying properties in a magnetic field.
The current carrying properties of crystallographically aligned layers of oxide superconductor are dependent on magnetic field orientation. FIG. 1 shows the typical field dependence of a metal-organic deposited (MOD) Y123 film on an oxide-buffered metal substrate with magnetic field oriented parallel and perpendicular to the planar face of the film. At both 27K and 75K, with the magnetic field oriented perpendicular to the planar face of the film, there is a significant decrease in Ic from the value in parallel orientation, limiting the usefulness of the Y123 wires in many coil applications. Many anticipated applications are planned for temperatures in the 55 to 65K region, in magnetic fields of 1-3 Tesla oriented perpendicular to the planar face of the film, which are conditions at which performance drops significantly. In addition to the parallel and perpendicular performance of the Y123 wires in magnetic field, it is important to examine the field performance at intermediate angles as shown in FIG. 2. As seen in FIG. 2, Y123 films typically show a small peak in the c-axis (0° and 180° or perpendicular to the planar face of the Y123 film), which can be enhanced through the presence of extended planar or linear defects (e.g., twin boundaries, grain boundaries, a-axis grains).
In many applications, e.g., motors and magnetic coils, HTS wires will experience local variations in the magnetic field orientation, so that the magnetic field experienced in one region of the wire can be quite different from the magnetic field experienced in another wire region. In such applications, the Y123 wire performance is determined by the minimum performance at any magnetic field orientation, and not solely by that at the perpendicular orientation. Thus, the HTS wire demonstrates reductions in current density in regions where the magnetic field deviates from an optimum orientation.